A waveguide-integrated optical detector (photodetector), as is known, is a photodetector which detects the intensity of light propagating along a waveguide integrated therewith, and provides an electrical signal indicative thereof. There are numerous techniques known in the art for making such photodetectors.
One technique common in the art of waveguide integrated photodetectors is leakage-based or "evanescent-coupled" detection. In a leakage-based detector, input light is guided along a waveguide located on the device (i.e., waveguide layers, having a lower cladding layer and a core layer) and a detector layer, made of a material having an energy bandgap smaller than that of the energy of the light being detected, is grown adjacent to the core of the waveguide. Light propagating along the waveguide core layer leaks into the detector layer which, because the energy of the light is greater than the bandgap of the detector layer, is absorbed by the detector layer generates electron hole pairs therein which are detected by electrodes disposed on the surface of the detection layer, as is known.
One problem with leakage-based detection is that the detector is not exposed to the peak intensity of the light because the peak intensity only exists in the central portion of the waveguide layers, not in the detection portion. Consequently, less than optimum optical signal detection and slower response time occurs. Also, because detection is based on the amount of light that leaks from the waveguide, this technique is very sensitive to the spatial mode distribution of the light.
Another technique known in the art of waveguide integrated photodetectors is that of "regrown" or "butt-coupled" detection. This technique also provides waveguide layers which propagate light along the device grown above a substrate. At a receiving end of the waveguide, a portion of the waveguide layers is etched away and detector layers are grown, thereby allowing the light to propagate directly (head-on) into the detector layers. This technique avoids the problems of leakage based detection by allowing the peak intensity to be incident directly on the detector layers. The process of growing the detector layers on the same substrate after etching away the waveguide layers is known as "regrowth" upon the original substrate. Regrowth results in a lower quality of growth because it is the second time layers have been grown from the same substrate. Consequently, "regrowth" detection layers provide lower quality optical detection. A similar result occurs if the detection layers are grown first and then part of the detection layers are etched away and the waveguide layers are grown (or regrown) on top of the substrate. In that cased the regrowth of the waveguide layers produces a low quality growth, thereby resulting in a lossy waveguide, less than optimum optical detection, and a longer, more difficult and more expensive fabrication process.
A third known optical detection technique provides optical detector layers grown on one (e.g., top) surface of a substrate and waveguide layers grown on the other (e.g., bottom) surface of the substrate having one end of the substrate (directly opposite the detector) etched away (i.e., an etched mirror) at a predetermined angle, thereby providing a reflective surface, such that the light travelling along the waveguide layers is reflected upwardly through the substrate and into the detector layers where the light is detected. Such a technique is described in the articles: R.J. Deri et al, "Low-Loss Monolithic Integration of Balanced Twin-Photodetectors with a 3dB Waveguide Coupler for Coherent Lightwave Receivers" IEEE Photonics Technology Letters, Vol. 2, No. 8 (August 1990), and R. Trommer, "Monolithic InGaAs Photodiode Array Illuminated through an Integrated Waveguide", Electronics Letters, Vol. 21, No. 9 (April 1985).
This technique requires at least two growth steps because the detector layers and the waveguide layers are grown on opposite sides of the substrate. More specifically, the detector layers are grown on one side of the substrate in a first growth process, and the waveguide layers are grown on the other side of the substrate in a second growth process. Thus, this is a cumbersome, time consuming procedure. Furthermore, because the light must be reflected through a thick, non-absorbing substrate wafers the light reflected from the etched mirror will diverge (due to the large thickness of .the substrate) before reaching the detector layers on the opposite surface of the substrate, thereby precluding the use of extremely small detectors.
Thus, it would be desirable to design a waveguide integrated optical detector that does not suffer from high optical loss or beam-spread and that allows for small optical detectors and does not require two-sided substrate growth.